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Ultrathin Aluminosilicate Films from Langmuir-Blodgett Multilayers C. M. Jones, M. Kalaji, J. A. Rees, and D. M. Taylor* Institute of Molecular and Biomolecular Electronics, University of Wales, Dean Street, Bangor, Gwynedd LL57 1 UT, U.K. Received July 30, 1997. In Final Form: November 3, 1997X Multilayer films incorporating both silicon and aluminum have been prepared by Langmuir-Blodgett deposition and subsequently decomposed by exposure to ultraviolet ozone (UVO) to yield films with a composition consistent with the formation of an aluminosilicate. The morphology of the deposited films was studied by atomic force microscopy both before and after UVO treatment, and their chemical structures and compositions were investigated by FTIR and XPS. Two film systems were investigated, namely, cyclic polysiloxanes with carboxylic acid side groups and octadecyltrimethoxysilane (OTMS). Aluminum was introduced into the films from a 10-5 M aluminum nitrate subphase at pH ∼ 4 in the first case and at pH ) 9.5 in the second. It is shown that uptake of aluminum and control of aluminosilicate formation is likely to be easier with the OTMS system. It is also shown that inclusion of an ether linkage in the hydrophobic tails of the polysiloxanes leads to a much more rapid removal of carbon from the films during UVO treatment. Preliminary work has been carried out on the inclusion of a precursor template molecule to control the structure of the aluminosilicate.
1. Introduction Thin metal oxide films have been fabricated using many different methods including thermal oxidation, sputtering, plasma and chemical vapor deposition, electron-beam evaporation, and the sol-gel technique. Recently, it has been shown that Langmuir-Blodgett (LB) films may be suitable precursors for ultrathin oxide films. For example, thermal processing of LB films has been used for the preparation of yttrium oxide1 and titanium oxide films.2,3 Cadmium oxide and cadmium carbonate films have been formed by an oxy-plasma treatment and then used as diffusion sources in GaAs devices.4 Attempts at pyrolyzing stearates and arachidates of divalent metals simply lead to melting of the film and droplet formation on the substrate surface before decomposition is achieved.5,6 However, erbium and cadmium arachidates were successfully decomposed by ultraviolet ozone (UVO) processing7 followed by thermal treatment. UVO treatment is widely used in the semiconductor industry for removing hydrocarbon contamination from silicon wafers, where it has the advantage of being an inexpensive, nonvacuum, low-temperature process. However, one of the main disadvantages is that less volatile inorganic residues are often created. It is this feature that is used in the preparation of metal oxides from LB films. The composition of some LB films is similar to the starting mixture in the standard sol-gel technique for X Abstract published in Advance ACS Abstracts, December 15, 1997.
(1) Amm, D. T.; Johnson, D. J.; Laursen, T; Gupta, S. K. Appl. Phys. Lett. 1992, 61, 522. (2) Paranjape, D. V.; Sastry, M.; Ganguly, P. Appl. Phys. Lett. 1993, 63, 18. (3) Sastry, M.; Pal, S.; Paranjape, D. V.; Ganguly, P. J. Electron Spectrosc. Rel. Phenom. 1994, 67, 163. (4) Shah, D. M.; Chan, W. K.; Bhat, R.; Cox, H. M.; Schlotter, N. E.; Chang, C. C. Appl. Phys. Lett. 1990, 56, 2132. (5) Taylor, D. M.; Lambi, J. N. Thin Sol. Films 1994, 243, 384. (6) Tippermann-Krayer, P.; Mo¨hwald, H.; Schrech, M.; Gopel, G. Thin Sol. Films 1993, 232, 245. (7) Amm, D. T.; Johnson, D. J.; Matsuura, M.; Palmer, G. Thin Sol. Films 1994, 242, 74.
the preparation of metal oxides,8 the final composition and structure of the oxide being determined by the firing conditions and subsequent anneal. While LB films have been used as precursors for the preparation of oxide films, their potential for forming ceramic and zeolite-type structures does not seem to have received much attention. This is surprising since ultrathin ceramic films could have important applications in the electronics industry. Furthermore, zeolites are being widely applied in ion exchange, adsorption, and catalysis.9 Current indications are that the use of zeolites in pollution control and bulk separation will escalate, but unfortunately, their microcrystalline structure (crystallite size of typically 0.1-200 µm) imposes a limitation on their applicability. As a result, some effort has been directed at forming thin zeolitic membranes by, for example, hydrothermal synthesis onto porous supports.10-12 Further details of zeolite-based membranes including their different methods of synthesis can be found in the review article by den Exter et al.13 Clearly, an ability to produce ultrathin zeolite films, approximately a few nanometers thick, will offer a significant advantage in molecular separation applications over conventionally produced films, which at best are about 3 orders of magnitude thicker. The present work was undertaken to explore the feasibility of preparing ultrathin zeolite films of controlled composition and uniform thickness from an LB precursor. As a first step in this investigation, the deposition and subsequent decomposition of LB films containing appropriate constituents, i.e., Al and Si and/or SiO, were (8) Hussain, A. A.; Sayer, M. J. Appl. Phys. 1991, 70, 1580. (9) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic: New York, 1978. (10) Geus, E. R.; van Bekkam, H.; Bakker, W. J. W.; Moulijn, J. A. Microporous Mater. 1993, 1, 131. (11) Anderson, M. W.; Pacis, K. S.; Shi, J.; Carr, S. W. J. Mater. Chem. 1992, 2, 255. (12) Myatt, G. J.; Mudd, P. M.; Price, C.; Carr, S. W. J. Mater. Chem. 1992, 2, 1103. (13) den Exter, M. J.; Jansen, J. C.; van der Graaf, J. M.; Kapteijn, F.; Moulijn, J. A.; van Bekkam, H. Stud. Surf. Sci. Catal. 1996, 102, 131.
S0743-7463(97)00846-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
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Figure 1. Cyclic polysiloxane molecules investigated in this work.
studied in some detail. Two systems were investigated: (i) cyclic polysiloxanes of general structure ((CH3)Si(R)O)4, with R being either an aliphatic or an aromatic chain terminated in a carboxyl moiety; and (ii) octadecyltrimethoxysilane. In the first case, aluminum was incorporated into the films by depositing LB films from an aluminum-containing subphase held at pH > 3.8 to form an aluminum salt. In the second case, LB films were deposited from an aluminum-containing subphase at pH > 9.0 when the methoxysilane groups are expected to be hydrolyzed.14 Preliminary experiments were also carried out investigating the possibility of incorporating template and precursor template molecules, tetramethylammonium salts and tetraoctadecylammonium bromide, respectively, into the LB films with a view ultimately to control the structure of the zeolite.15,16 2. Experimental Section The synthesis of the cyclic polysiloxanes 1-3 (Figure 1), supplied by D. Lacey, Hull University, has been described elsewhere.17 Octadecyltrimethoxysilane (OTMS), tetraoctadecylammonium bromide (TOAB), dimethyldioctadecylammonium bromide (DDAB), tetramethylammonium hydroxide (TMAOH), and tetramethylammonium iodide (TMAI) were obtained from Aldrich Chemical Co. LB films were deposited from a sliding barrier poly(tetrafluoroethylene) (PTFE) trough located on an antivibration table in a Class II semiconductor clean room. Ultrapure water for the trough and for the preparation of reagents (all Analar grade) was obtained from a Millipore RO60 system comprising reverse osmosis, ion exchange, organex, and 0.2-mm filter cartridges. The surface pressure was continuously monitored using a Wilhelmy plate and electrobalance arrangement. Experiments were performed with the subphase held at room temperature (20 ( 2 °C) and with the pH adjusted using either HCl or NaOH. Spreading solutions of polysiloxanes 1 and 3 were prepared by dissolving 0.64 and 1.1 mg/mL, respectively, in chlorofrom (HPLC grade). Polysiloxane 2 was dissolved at the rate of 0.6 mg/mL in a 1:1 (by volume) mixture of chloroform and 2-ethoxyethyl acetate. Monolayers were formed by spreading 75-100 mL of these solutions onto the subphase surface, allowing at least 10 min for the solvent to evaporate. In an earlier study,18 we established that aluminum-containing LB films of the polysiloxanes may be deposited successfully from an aluminum (14) Linden, M.; Slotte. J. P.; Rosenholm, J. B. Langmuir 1996, 12, 4449. (15) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic: London, 1982. (16) Zeolite Synthesis; Ocelli, M. L., Robson, H. E., Eds.; ACS Symposium Series: American Chemical Society: Washington, DC, 1989. (17) Abd Majid, W. H.; Richardson, T.; Holder, S.; Lacey, D. Thin Sol. Films 1994, 243, 378. (18) Jones, C. M.; Kalaji, M.; Rees, J. A.; Taylor, D. M.; Richardson, T.; Lacey, D. Supramolec. Sci. 1997, 4, 335.
Jones et al. nitrate subphase, concentration 12.0. At extreme values of pH, the monolayer may be fully polymerized in minutes.14 Monolayers were prepared by spreading 75-100 mL of a solution of OTMS in chloroform (0.883 mg/mL) onto the subphase surface, and at least 10 min was allowed for solvent evaporation. Experiments were carried out for a range of subphase pHs and aluminum nitrate concentrations. Monolayer transfer was effected onto metallized glass and mica substrates as well as onto hydrophobic silicon. The metallized substrates were prepared by thermal evaporation of gold or aluminum (99.99% purity from Advent Research Materials) in a turbo-pumped system. Gold layers were keyed onto substrates with a 10-20-Å chromium layer. Mica slides (Agar Scientific) were freshly cleaved prior to metallization, while glass slides were cleaned by ultrasonication in Decon 90, thoroughly rinsed with ultrapure water, and then dipped in concentrated nitric acid for 5 min prior to a final rinse in ultrapure water. The slides were then dried in a stream of dry nitrogen. The silicon substrate was a highly polished, n-type silicon wafer preoxidized with an 8500-Å-thick oxide layer. The wafer was cleaned by placing it in a hot solution of 10 mL of ammonia, 5 mL of hydrogen peroxide, and 85 mL of water for 5 min, followed by 5 min in a hot solution of 5 mL of HCl, 10 mL of hydrogen peroxide, and 85 mL of water and a final rinse in ultrapure water. The resulting hydrophilic oxide was rendered hydrophobic by placing the wafer in the vapor of 1,1,1,3,3,3-hexamethyldisilazane (Aldrich Chemical Co.) overnight. The optimum dipping pressures and speeds were different for different materials but were generally about 25 mN/m and 4-8 mm/min, respectively. The deposited films were examined for gross defects by scanning surface potential measurements19 using a Trek Model 320B vibrating plate voltmeter and by phase contrast microscopy. Microscopic characterization of the films was undertaken using a Nanoscope IIIA atomic force micrscope (AFM) operating in the tapping or contact mode, as appropriate. The film composition was investigated using grazing angle FTIR spectroscopy (Bruker, IFS113v) using p-polarized radiation at 4-cm-1 resolution, X-ray photoelectron spectroscopy (XPS) using a VG220i instrument (Al KR radiation, operating at 180 W of power and a photoelectron takeoff angle of 60° unless otherwise stated), and time-of-flight secondary ion mass spectroscopy (TOF SIMS) carried out under static conditions using a VGIX233 instrument (pulsed liquid metal ion source, Ga+, 30 keV, and a pulsed electron source for charge compensation) equipped with a Poschenrieder TOF analyzer. The XPS and TOF SIMS measurements were carried out by CSMA Ltd. (Manchester, U.K.). Decomposition by UVO treatment was performed with samples placed 8 cm from the UV source of a commercial unit (UVOCS Model 0306E) kindly loaned by Megatech Ltd. (Havant, Hants, U.K.). Attempts at thermal decomposition were made in a quartz furnace tube through which an O2 or N2 flow was maintained.
3. Results and Discussion 3.1. Polysiloxanes. Figures 2 and 3 show the isotherms obtained for polysiloxanes 1 and 2 on ultrapure water (pH ) 5.6) and 10-5 M aluminum nitrate subphases at lower pHs. On ultrapure water, the isotherms were similar to those reported by Abd Majid et al.17 with a clear phase change occurring in the range 15-25 mN/m. Figure 4 contains isotherms obtained for polysiloxane 3 on ultrapure water and aluminum nitrate subphases at pH ) 3.4 and 5.1. This shorter chain molecule is more (19) Oliveira, Jr., O. N.; Taylor, D. M.; Stirling, C. J. M.; Tripathi, S.; Guo, B. Z. Langmuir 1992, 8, 1619.
Ultrathin Aluminosilicate Films
Figure 2. Surface pressure-area (π-A) isotherms of polysiloxane 1 on ultrapure water at pH 5.6 (s) and on a 10-5 M aluminum nitrate subphase at pH 4.5 (2) and pH 3.1 (O).
Figure 3. Surface pressure-area (π-A) isotherms of polysiloxane 2 on ultrapure water, at pH 5.6 (s) and on a 10-5 M aluminum nitrate subphase at pH 3.9 (2) and pH 3.1 (O).
expanded at low surface pressures. Between about 10 and 20 mN/m, it follows closely the isotherm for polysiloxane 1 but, upon further compression, continues to rise smoothly until collapse gradually sets in above about 30 mN/m. In all cases, the isotherms obtained on a 10-5 M aluminum nitrate subphase for pH e3.4 were identical to those obtained on ultrapure water. As the subphase pH was increased above ∼3.4 for polysiloxane 1 and ∼3.8 for polysiloxane 2, the phase transition was lost and the isotherms rose more steeply at higher pressures. The isotherm for polysiloxane 3 also rose more steeply at the higher pH. The pKa of stearic acid on an aluminum nitrate subphase has been estimated to be in the range 3.4-3.8.20 We may presume, therefore, that the changes seen here are caused by ionization of the carboxylic acid headgroups and the formation of the aluminum salt. (20) Binks, B. P. Adv. Colloid Interface Sci. 1991, 34, 343.
Langmuir, Vol. 14, No. 2, 1998 381
Figure 4. Surface pressure-area (π-A) isotherms of polysiloxane 3 on ultrapure water, at pH 5.6 (s) and on a 10-5 M aluminum nitrate subphase at pH 5.1 (2) and pH 3.4 (O).
As reported earlier,18 monolayers of the polysiloxanes 1 and 2 could be deposited on both the up- and downstroke with deposition ratios of 0.9-1.0 and 0.8-0.9, respectively, at a dipping speed of 4 mm/min. Both FTIR and XPS demonstrated clearly the takeup of aluminum in films deposited from an aluminum nitrate subphase.18 Similar success was obtained in the deposition of polysiloxane 3. The initial attempts at thermal decomposition at temperatures ranging from 50 to 250 °C all failed, with films melting and coalescing into droplets reminiscent of the behavior of divalent metal arachidates and stearates.5,6 In contrast, films that had undergone UVO treatment remained coherent and showed significant chemical changes as evidenced in the FTIR spectra of treated and untreated polysiloxane 2 in Figure 5. The carboxycarbonyl stretch at 1717 cm-1 is broadened and shifted to lower wavenumbers (cf. spectra a and c), indicating changes in the cyclic acid-dimer arrangement. Temperature-induced disorder and UV polymerization of unsaturated, long-chain carboxylic acids21,22 produce similar effects. The sharp, aromatic band18 at 1513 cm-1 disappears after 10 min of UVO treatment. A number of other bands are seen to increase or shift in wavenumber. The absence of any bands associated with the aromatic ring in spectra b and c suggests that the photolysis reaction has either cleaved the chain at the ether linkage or opened the aromatic ring. The latter reaction is not impossible and is reminiscent of the oxidative degradation of aromatic hydrocarbons by microorganisms.23 A possible mechanism is the attack on the benzilic carbon (b to the carboxylic acid) by singlet oxygen, resulting in the eventual formation of a cinnamic acid. Further attack is then possible on the alkene double bond, destabilizing the whole molecule and yielding the half ester of oxalic acid, which would be consistent with the presence of the broad band at ∼1700 cm-1. An interesting feature of spectra b and c, however, is the development of a doublet centered around 1347 cm-1 (21) Davies, G. H.; Yarwood, J.; Petty, M. C.; Jones, C. A. Thin Sol. Films 1988, 159, 461. (22) Guo, B. Z.; Tripathi, S.; Taylor, D. M.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1991, Issue 7, 479. (23) Gibson, D. T.; Koch, J. R.; Kallio, R. E. Biochemistry 1968, 7, 2653.
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Table 1. Change in Composition of 10- and 20-Layer Films of Polysiloxane 1 with UVO Treatment composition, atomic % 10-layer film
20-layer film
element
t ) 0 min
t ) 5 min
t ) 10 min
t ) 0 min
t ) 60 min
C O Si Al Au
80.6 ( 0.9 13.0 ( 0.6 5.4 ( 0.7 0.9 ( 0.3 0.13 ( 0.02
78.1 ( 0.8 15.7 ( 0.6 5.3 ( 0.6 0.8 ( 0.3 0.08 ( 0.02
76.1 ( 0.8 17.1 ( 0.5 5.6 ( 0.6 1.0 ( 0.3 0.27 ( 0.02
82.1 ( 0.4 12.4 ( 0.4 5.3 ( 0.3 0.3 ( 0.2 0.01 ( 0.01
62.0 ( 0.6 26.7 ( 0.5 10.9 ( 0.5 0.4 ( 0.2 0.1 ( 0.02
Table 2. Change in Composition of a 20-Layer Film of Polysiloxane 2 with UVO Treatment composition, atomic % element C O Si Al N Au
t ) 0 min 70.9 ( 0.5 20.5 ( 0.4 7.9 ( 0.4 0.6 ( 0.2 0.02 ( 0.01
t ) 20 min
t ) 60 min
41.7 ( 0.8 41.3 ( 0.7 14.1 ( 0.8 0.8 ( 0.3 1.9 ( 0.6 0.2 ( .02
20.7 ( 1.2 50.6 ( 1.1 22.0 ( 1.2 1.7 ( 0.4 1.2 ( 0.6 3.9 ( 0.1
Table 3. Change in Composition of a 36-Layer Film of Polysiloxane 3 with UVO Treatment composition, atomic % element C O Si Ala N Au
t ) 0 min 54.8 ( 0.5 31.4 ( 0.4 11.5 ( 0.4 1.2 ( 0.2 0.04 ( 0.01
t ) 20 min
t ) 40 min
t ) 60 min
24.1 ( 0.9 47.1 ( 0.8 20.4 ( 0.8 2.6 ( 0.4 1.0 ( 0.4 1.9 ( .1
14.5 ( 0.9 53.5 ( 0.8 21.2 ( 0.7 1.9 ( 0.4 1.8 ( 0.5 2.3 ( 0.1
14.1 ( 0.8 53.5 ( 0.8 23.0 ( 0.7 2.5 ( 0.4 2.1 ( 0.5 1.7 ( 0.1
a
Al values corrected to account for Au overlap and are minimum values.
which only occurs when ether oxygen is present in the hydrocarbon chain of the polysiloxanes. We attribute this feature to the presence of nitrate ions in the film. It should be noted that, although the LB films were deposited from an aluminum nitrate subphase, no trace of nitrate ions was seen in the untreated films. The nitrate ions were only detected after photolysis, and this is consistent with the appearance of nitrogen in the XPS spectra of treated films (see below). The presence of nitrates in these particular films suggests that the mechanisms of photolysis are quite complex. From atmospheric chemistry,24,25 UV ionization of N2 and O2 mixtures is expected to produce NOx+, which may then react with the ether oxygen to form nitrate. Further quantitative work using, for example, on-line GCmass spectrometry would be necessary, though, to confirm this hypothesis.
The bands between 1000 and 1260 cm-1 can be assigned to the symmetric and asymmetric stretches of Si-O-Si. Similar bands have been reported for alkoxysilanes.26 The slight changes in the structure and position of these bands may indicate a change from organic to inorganic species. Films of polysiloxane 1 showed a gradual reduction in the intensity of all bands but with the relative decrease in the C-H stretching and bending modes being much greater than for the Si-O modes. This again is consistent with fragmentation and gradual loss of the aliphatic chain. No FTIR spectra were obtained for polysiloxane 3. The evidence presented above for chemical changes in the films was supported by XPS analysis. As can be seen from Tables 1-3, the chemical compositions (atomic %) of deposited films changed progressively with increasing UVO treatment time. After an hour of UVO exposure, a clear feature of all the data is the large decrease in carbon content of films down to 62%, 20.7%, and 14.1% in the three polysiloxanes. Not surprisingly, this is accompanied by large increases in the relative concentrations of oxygen and silicon. Some increase in the aluminum content is also observed. During these changes, the proportion of oxidized carbon functionality increased as evidenced from XPS by the change in relative magnitudes of the overlapping CsC and CsH peaks at a binding energy of ∼285 eV and the OsCdO peak at ∼289.5 eV. That the silicon peak position was observed at 103.2 eV for this sample is consistent with the presence of an oxidized form such as silica or silicate. A peak at 75 eV confirmed the presence of oxidized aluminum. Nitrogen was detected in polysi-
(24) The Photochemistry of Atmospheres; Levine, J. S., Ed.; Academic: Orlando, FL, 1985. (25) Hirst, D. The Computational Approach to Chemistry; Blackwell: Oxford, 1990.
(26) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 3696. (27) Taylor, D. M.; Gupta, S. K.; Dynarowicz, P. Thin Sol. Films 1996, 284-285, 80.
Figure 5. Grazing incidence FTIR spectrum (a) of an untreated 10-layer film of polysiloxane 2 deposited from an aluminum nitrate subphase at pH 4.0. Shown for comparison are spectra obtained for the same film after (b) 10 min and (c) 20 min of UVO exposure.
Ultrathin Aluminosilicate Films
Langmuir, Vol. 14, No. 2, 1998 383
Figure 6. Depth profile of the elemental composition of a 60min UVO-processed 20-layer film of polysiloxane 2 obtained by changing the electron takeoff angle (ETOA) in high-resolution XPS. The films were deposited onto a gold substrate from a 10-5 M aluminum nitrate subphase at pH 3.9.
loxanes 2 and 3 after UVO treatment, consistent with the appearance of nitrate ions in the FTIR spectra. The signal from the underlying gold substrate in Tables 1-3 increased with increasing UVO processing time, indicating a thinning or desorption of the film. Angleresolved XPS studies on polysiloxane 2 suggested a progressive thinning of the film rather than the loss of large areas since the gold signal decreased monotonically with decreasing photoelectron takeoff angle (Figure 6). The increase in the concentrations of silicon and carbon as the electron takeoff angle decreases is greater than would be expected from the loss of the gold signal, suggesting a slight increase in the concentrations of carbon and silicon in the upper layers of the film. Angle-resolved measurements on an unprocessed 20layer film of polysiloxane 2 give a lower limit of 8 nm for the film thickness (expected thickness ∼28 nm). Following 20 min of UVO treatment, the appearance of the gold signal at the lowest takeoff angle suggests that the film had thinned down to about 3 nm. Figures 7 and 8 are AFM images of polysiloxanes 1 and 2 respectively both before and after UVO processing. Asdeposited, 10-layer films of polysiloxane 1 formed elliptical islands (Figure 7), with their major axes ranging from 0.1 to 0.5 µm in length. UVO processing only gradually eroded these structures, the RMS roughness decreasing from 7.95 nm in the unprocessed film to 3.34 nm after a 20-min exposure. A 10-layer, as-deposited film of polysiloxane 2, although composed of almost circular islands ∼50 nm in diameter (Figure 8), is generally smooth with an RMS roughness of less than 2.0 nm. After 20 min of UVO processing, the island structure breaks up, yielding a film of uniform appearance over large areas with RMS roughness < 0.4 nm. No evidence of patchy deposition or nonuniform erosion was seen, in keeping with the XPS results. Polysiloxane 3 was composed of small, irregularly shaped islands approximately 50 nm in size. The RMS roughness of a five-layer film was ∼1.1 nm, reducing to 0.91 nm after 9 min of UVO processing. Although the structures of the polysiloxanes differ in detail, the effect of UVO processing in all cases is to break down the island structure and reduce both the thickness of the film and its RMS roughness. Building up films by alternating the deposition of a small number of layers with UVO treatment did not appear to
Figure 7. AFM image of a 10-layer film of polysiloxane 1 deposited onto hydrophobic silicon from a 10-5 M aluminum nitrate subphase at pH 3.5. Image A was obtained for the untreated film. Typical images obtained after (B) 10 min and (C) 20 min of UVO processing are also shown.
change the atomic composition of the final film to any significant degree. Subsequent heating of films to 150 °C for 1 h in O2 produced no further changes in composition.
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Figure 9. Surface pressure-area (π-A) isotherm of OTMS on ultrapure water at pH 5.6 (s) and when the pH is adjusted to 9.5 (O) and 12.0 (2) using NaOH.
Figure 8. AFM image of a 10-layer film of polysiloxane 2 deposited onto hydrophobic silicon from a 10-5 M aluminum nitrate subphase at pH 3.9. Image A was obtained for the untreated film and (B) after 20 min of UVO processing.
As observed by Amm and coworkers,1 UVO treatment seemed to prevent film melting and droplet formation, confirming that a significant fraction of the organic content of the film had been lost. Unsuccessful attempts were made to incorporate TMAOH into the films by adding the template to the aqueous subphase prior to dipping. In further experiments, both processed and unprocessed films were dipped into aqueous solutions of the template. No uptake of the template was detected in any of these experiments. In a final attempt, N2 was bubbled through a solution of TMAOH and passed over deposited films placed in a glass container. Again, no uptake of the template was detected. The above experiments show that LB films formed from aluminum salts of polysiloxanes 1-3 show promise as potential precursor systems for the preparation of ultrathin aluminosilicate films. However, further work is necessary to develop methodologies (i) for completely removing the carbon from the films and (ii) for incorporating templates to control the structure of the films produced. 3.2. Octadecyltrimethoxysilane. The isotherms of OTMS on a pure water subphase at pH 5.6 and on alkaline subphases at pH ) 10.0 and 11.6 (see Figure 9) are
identical to those reported earlier.14,27 For pH e 10, the transition from liquid-expanded to liquid-condensed phases is clearly seen at ∼9 mN/m, as is the secondary transition at ∼21 mN/m. At higher pHs, the monolayer condenses at much lower surface pressures, the phase transitions disappear, and the isotherm rises monotonically to collapse. The condensed-phase area per molecule is slightly greater at high pHs, consistent with the notion that, at intermediate pH values, the area per molecule is dominated by the hydrocarbon chain, while at high and low pHs, the headgroup dominates. At pH 12.0, the monolayer was significantly more rigid because of hydrolysis and polycondensation of the methoxy headgroup.14 The resulting polymeric structure would prevent the vertical “slipping” of the headgroups which has been suggested27 as the origin of the secondary phase transition seen at ∼20 mN/m at pH 5.6 in Figure 10. Unfortunately, the rigid films produced at high pHs resulted in poor multilayer deposition, even when attempted within a few minutes of spreading. In an effort to reduce film rigidity and simultaneously introduce a template molecule into the monolayer to direct the polymerization, mixtures of OTMS with DDAB and TOAB were investigated. 3.2.1. Mixtures of OTMS and DDAB. Mixtures of OTMS and DDAB in the molar ratios 4:1, 2:1, and 1:1 formed stable monolayers on a pure water subphase at pH 12.0 as evidenced by the isotherms in Figure 10, produced ∼15 min after spreading. For ease of comparison, the molecular areas are expressed in terms of the area per OTMS molecule in the mixture. Thus, the lateral shifts between the isotherm of pure OTMS (continuous curve) and those of the mixtures represent the area taken up by the template molecules, in this case DDAB. Assuming that both octadecyl chains of the DDAB molecules are vertically orientated, we would expect a shift of approximately 40 Å2 per OTMS molecule in the 1:1 mixture, i.e., approximately double that observed experimentally. This suggests that each DDAB molecule is held in the structure with one octadecyl chain orientated vertically. The relative shifts seen at the highest surface pressures for the 2:1 and 4:1 OTMS:DDAB mixtures are
Ultrathin Aluminosilicate Films
Langmuir, Vol. 14, No. 2, 1998 385 Table 4. Conditions under Which the Various 4:1 OTMS:TOAB Multilayers were Deposited sample
pH
Al(NO3)3 concn, M
spreading time, min
TP16 TP20 TP21a TP22 TP33 TP34
9.50 9.70 9.70 11.60 9.70 8.80
1.3 × 10-6 1.0 × 10-5 1.0 × 10-5 1.3 × 10-6 1.0 × 10-5 1.0 × 10-5
10 10 910 10 60 10
a This sample was deposited at a later time from the same spread monolayer as TP20.
Table 5. Composition of 20-Layer Films of a 4:1 OTMS:TOAB Mixture as Determined by XPS
Figure 10. Surface pressure-area (π-A) isotherm of pure OTMS (s) on ultrapure water adjusted to pH 12.0 using NaOH. Also shown are the isotherms obtained for mixtures of OTMS: DDAB in the mole ratios 4:1 (O), 2:1 (2) and 1:1 (1).
Figure 11. Surface pressure-area (π-A) isotherm of pure OTMS (O) and a 4:1 OTMS:TOAB mixture (2) on ultrapure water adjusted to pH 9.5 using NaOH. Also shown (0) is the isotherm obtained for the 4:1 mixture on a 10-5 M aluminum nitrate subphase.
approximately 10 and 5 Å2, respectively, supporting this hypothesis. Unfortunately, attempts at depositing these films at pH 12.0 by conventional vertical dipping were unsuccessful even before adding aluminum nitrate to the subphase, so no further work was undertaken with this mixture. 3.2.2. Mixtures of OTMS and TOAB. Figure 11 compares the isotherms of pure OTMS (O) and a 4:1 OTMS: TOAB mixture (2) on an ultrapure water subphase at pH 9.5. At the highest surface pressures, the increase in area per OTMS molecule for the mixture is ∼27 Å2, suggesting that the aliphatic chains of TOAB are fully incorporated into the monolayer. At pH 9.5, addition of aluminum nitrate to the subphase caused the isotherm (O) to expand at low surface pressures,
sample
C:Si
C:O
O:Si
Al:Si
TP16 TP20 TP21 TP22 TP33 TP34
36.6 ( 3.1 39.8 ( 3.8 45.1 ( 6.2 42.4 ( 6.0 40.3 ( 4.3 44.8 ( 5.0
16.3 ( 1.0 9.8 ( 0.4 2.9 ( 0.1 16.7 ( 1.3 6.0 ( 0.2 8.3 ( 0.2
2.3 ( 0.3 4.1 ( 1.0 15.7 ( 2.0 2.6 ( 0.6 6.8 ( 0.8 6.1 ( 0.8
0.2 ( 0.1 0.8 ( 0.2 5.7 ( 1.0 0.2 ( 0.1 2.1 ( 0.3 1.7 ( 0.2
Table 6. Expected Composition of 4:1 OTMS:TOAB Multilayers assuming the OTMS To Be Unpolymerized and Fully Polymerized into an Si-O-Si Network OTMS
C:Si
C:O
O:Si
unpolymerized fully polymerized
39 36
13 24
3 1.5
Al:Si
but at ∼25 mN/m, a transition to a more condensed phase occurred, similar to that seen for pure OTMS at lower pHs. The presence of aluminum appears, therefore, to inhibit polymerization of the OTMS headgroup. While 4:1 mixtures of OTMS:TOAB were successfully deposited from an ultrapure water subphase at pH 12.0, addition of aluminum nitrate to the subphase again resulted in a film that proved too rigid to deposit by conventional vertical dipping. Deposition from an aluminum nitrate subphase was possible at pH 9.5 (deposition pressure > 25 mN/m, deposition speed ) 4 mm/min for 2 layers and then 8 mm/min thereafter), but deposition ratios of between 0.35 and 0.55 for both the up- and downstrokes were much lower than obtained for the cyclic polysiloxanes. In principle, the degree of polymerization of the OTMS in the mixed multilayer and the uptake of aluminum may be deduced from the atomic compositions of the deposited films. Hence, a series of experiments were conducted in which the monolayer-forming conditions were varied systematically and the composition of 20-layer films determined by XPS. Table 4 gives the details of pH, subphase aluminum concentration, and spreading time for each of the samples. Samples TP21 and TP22 were too rigid to be deposited by conventional vertical dipping but were deposited successfully using an alternating vertical/horizontal dipping technique developed by Batty et al.28 The atomic compositions of the various samples, expressed in terms of the atomic ratios of the elements present, are given in Tables 5 and 6 together with the expected values calculated assuming that the 4:1 OTMS: TOAB mixture is either unpolymerized or fully polymerized. In the latter case, all the methoxy groups were assumed to be hydrolyzed and polycondensed to form an -O-Si-O-Si- network. The calculations excluded the possibility of hydroxyaluminum complexes being associated with the monolayer. (28) Batty, S. V.; Richardson, T.; Pocock, P.; Rahman, L. Thin Sol. Films 1995, 266, 96.
386 Langmuir, Vol. 14, No. 2, 1998
Jones et al. Table 7. Change in Composition of a 20-Layer Film of a 4:1 OTMS:TOAB (Sample Type TP33) with UVO Treatment composition, atomic %
Figure 12. Reduction in carbon concentration as measured by high-resolution XPS following UVO exposure of the various films. The curves refer to polysiloxanes 1 (2), 2 (1), and 3 (b) and the 4:1 OTMS:TOAB samples TP33 (0) and TP34 (O). The films were all 20 layers thick and deposited on gold.
The calculations suggest that a slightly lower C:Si ratio can be expected in polymerized films. Unfortunately, experimental error precludes the detection of such a small difference. In contrast, the C:O ratio is expected to almost double after polymerization, while the O:Si ratio is expected to halve. Comparison with experiment shows that TP16 and TP22 have the lowest oxygen content, suggesting the greatest degree of polymerization. These films were deposited from subphases with the lowest aluminum content. It is likely, therefore, that higher concentrations of hydroxyaluminum complexes may inhibit the hydrolysis and subsequent polycondensation of the methoxy groups, consistent with the surface pressure isotherms in Figure 11. Of the samples deposited from the 1 × 10-5 M subphase at pH ∼9.6, both the oxygen and aluminum content increased with increased spreading time on the trough, cf. samples TP20, TP21, and TP33. Clearly, as the spreading time increases, the reduction in oxygen content owing to polycondensation of the methoxy groups is being swamped by a strong associative interaction between the monolayer and the subphase hydroxyaluminum complexes. No XPS signal from the gold substrate was detected for any of these films except for TP21 and TP22, where it was
element
t ) 0 min
t ) 60 min
t ) 120 min
C O Si Al N Au
79.7 ( 0.4 13.4 ( 0.2 2.0 ( 0.2 4.1 ( 0.2 0.8 ( 0.4
59.4 ( 0.4 30.7 ( 0.3 3.2 ( 0.2 5.7 ( 0.2 0.8 ( 0.3
40.3 ( 0.6 43.3 ( 0.6 5.5 ( 0.5 8.2 ( 0.3 1.2 ( 0.4 0.9 ( 0.02
just detectable. It is significant that these were also the most rigid films and, therefore, the most prone to defects. No XPS signals corresponding to NaOH or from the Branion of TOAB were detected. Two distinct nitrogen peaks at ∼402 and ∼399 eV were observed, the former consistent with the presence of the quaternary TOAB and the latter with a range of organic compounds including amines, amides, and cyanides, which may have been trace contaminants in the original reagents. TOF SIMS analysis on two samples of type TP33 gave a number of negative ion signals which were assigned to aluminum-containing fragments of an oxide/oxide-like form with composition (AlxOy)(OH)z- and a number of positive ion signals consistent with the presence of poly(dimethylsiloxane). Unfortunately, it was not possible to establish the formation of an aluminosilicate owing to the overlap in mass of the Si-O and Al-O fragments. Positive ion micrographs from a sample of TP33 showed relatively uniform lateral distributions of aluminum and organic material over an area 300 µm × 300 µm. The reduction in the carbon content of two samples of type TP33 during UVO processing is plotted in Figure 12 and compared with the results obtained from the cyclic polysiloxanes. The decrease is approximately linear and almost identical to that found for polysiloxane 1, which has a similar hydrocarbon chain length. In common with the polysiloxanes, the oxygen content increased substantially during processing, while both the silicon and aluminum content also increased (see Table 7). XPS shows that the silicon chemistry changed during processing owing to the oxidation of organic silicone species to inorganic Si-O bonds found in silica and silicates. TOF SIMS carried out after processing showed the presence of TOAB fragments, thus confirming that the template had also decomposed during UVO processing. Heating the samples for up to 5 h at 50 °C produced little further change in the composition of the UVOprocessed film but resulted in a larger XPS signal from the gold substrate, indicative of a patchy film.
Figure 13. AFM micrographs of a 20-layer film of the 4:1 OTMS:TOAB mixture deposited onto gold-coated mica from a 10-5 M subphase at pH ) 10. The untreated film is shown in (a), while the effects 20 and 45 min of UVO processing are shown in (b) and (c), respectively.
Ultrathin Aluminosilicate Films
Langmuir, Vol. 14, No. 2, 1998 387
AFM micrographs of unprocessed OTMS:TOAB films show them to be composed of circular aggregates which increase in size from around 10 nm in the first monolayer deposited (RMS roughness ∼ 0.4 nm) to around 150 nm in a 20-layer film, as can be seen in Figure 13a. The RMS roughness increases with the number of layers and is ∼15 nm for the untreated 20-layer film. In some films, platelike structures were seen in addition to the circular aggregates. After UVO processing for 20 min, there was no obvious change in the morphology (Figure 13b), but after about 45 min, some regions appeared to be composed of more ordered aggregates (Figure 13c). After subsequent heating at 50 °C, the films became very patchy, confirming the XPS result above. These results show clearly that the OTMS:TOAB mixed monolayer has potential as a zeolite precursor. By controlling the subphase pH and monolayer spreading time, a degree of control may be exercised over the polymerization of the methoxysilane moieties and their interaction with hydroxyaluminum ions in the subphase. UVO processing significantly reduces the concentration of carbon in the films while simultaneously increasing the concentrations of silicon (as an inorganic oxide species) and aluminum (as an oxide). Further work is in progress to develop the methodology.
4. Conclusions An investigation has been undertaken into the feasibility of preparing ultrathin, aluminosilicate films from LB film precursors. Two systems were studied, namely, the cyclic polysiloxanes with carboxylic acid side groups and octadecyltrimethoxysilane. In the former case, aluminum was introduced into the monolayer by addition of aluminum nitrate to the subphase and lowering the pH to ∼4.0. In the latter case, aluminum nitrate was added to the subphase and the pH raised to ∼9.5. In both cases, FTIR and XPS showed that aluminum had been incorporated into the monolayer. UVO processing significantly reduced the concentration of carbon in the films while significantly increasing the silicon and aluminum concentrations. The presence of the ether linkage in polysiloxanes 2 and 3 accelerated the decomposition of the tailgroups, suggesting that a similar linkage would be beneficial in the decomposition of the long alkyl chain of the OTMS system. This and other possibilities are under active investigation. Acknowledgment. We are grateful to Dr. S. Armstrong and Dr. I. Hudson of BNFL for valuable discussions and the financial support of this work. LA970846P
